Direct thermal printable media is most widely used as facsimile paper but is also used in printers and in other applications requiring permanent imaging such as tags, tickets, and labels. In contrast to printing technologies that involve the transfer of ink from one location to another, direct thermal printing uses a special printable media that incorporates a color developing mechanism. Images are formed by exposing the color developing mechanism to concentrations of heat that produce localized chemical reactions involving a change of color (usually light to dark). The color developing mechanism is provided by a thermosensitive imaging material containing heat reactive chemicals such as leuco dyes or metallic salts.
Since direct thermal printing only involves the transfer of heat, printing of direct thermal media is simple and clean. Other advantages include low cost, low noise, and high speed. However, the thermosensitive imaging material is susceptible to damage from exposure to various environmental conditions including abrasion, heat, light, and chemicals such as oils, fats, blood, alcohol, solvent, and water. Much of this damage can be limited by applying protective coatings that block unwanted environmental interactions with the thermosensitive imaging material.
For example, U.S. Pat. No. 4,711,874 to Yuyama et al. suggests use of a water-soluble polymeric material as a protective overcoating for providing physical and chemical resistance. U.S. Pat. No. 4,717,709 to Suzuki suggests use of a polyolefin resin for similar purposes. U.S. Pat. No. 4,886,774 to Doi discloses a protective overcoating containing UV blockers. U.S. Pat. No. 5,286,703 to Wachi et al. discloses use of multiple protective overcoat layers including a first layer of water-soluble or water-insoluble polymers for chemical resistance and a second layer containing UV blockers.
The thermosensitive imaging material is applied as a coating to a substrate surface. Printing takes place by exposing the coating to a pattern of heat conducted from a thermal print head located adjacent to the substrate surface containing the coating of thermosensitive imaging material. The coating takes the form of the substrate surface including any irregularities or roughness in the surface. Accordingly, any surface irregularities in the thermosensitive coating vary spacing between the thermal print head and different points on the coating, causing unwanted dissipations of heat that interfere with image quality.
Two solutions are known to limit irregularities in the thermosensitive coating. One is to use only substrates with smooth surfaces. The other is to apply an undercoating between the substrate and the thermosensitive coating. The undercoating covers irregularities in the substrate surface and provides a smooth base for applying the thermosensitive coating. An example is found in U.S. Pat. No. 4,711,874 to Yuyama et al.
Protective overcoatings shield thermosensitive coatings from environmental interactions, and undercoatings provide a smooth base for applying thermosensitive coatings; but both add cost and complexity to direct thermal printable media. Also, the undercoatings can change the appearance or other desired properties of the substrates. However, without an undercoating, the choice of substrate is even more limited.